Add cross-segment Select with result_offset for wide table scans

[krleonid]

Summary

• Adds result_offset parameter to compression_select_t API, enabling Select to write at arbitrary positions in the result vector
• ColumnData::SelectVector now handles cross-segment selection by splitting the selection vector across segment boundaries
• StandardColumnData::Select no longer falls back to full Scan+Slice when vectors span multiple segments
• StructColumnData::Select propagates selection to child columns and short-circuits for constant-NULL structs

Problem

With 16KB block size, VARCHAR segments hold ~1279 rows. A 2048-row vector spans ~2 segments. Previously, StandardColumnData::Select checked scan_entire_vector which was always false for cross-segment vectors, forcing a fallback to Scan(all 2048 rows) + Slice(keep 34) — wasting 98% of decode work.

Solution

SelectVector now iterates over segments, calling segment.Select() with the appropriate result_offset for each segment's portion of selected rows. All compression Select implementations (RLE, uncompressed string, FSST, dict_fsst, validity, numeric constant, empty validity) accept the new offset parameter.

Benchmarks

DB1: 559 columns, 135K rows, 16KB blocks, dynamic filter selects ~34 rows per 2048-row chunk:

Metric	Baseline	Optimized	Change
Scan	0.115-0.128s	0.103-0.123s	-15%
Latency	0.186	0.171	-10%

DB2: 982 columns, 21K rows, 16KB blocks:

Metric	Baseline	Optimized	Change
Scan	0.148s	0.132s	-11%
Latency	0.326s	0.259s	-21%

Test plan

• test/sql/types/struct/* — all 44 tests pass
• Correctness verified (count + sum of column lengths match)
• Full CI suite

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